A. Mass Flowmeters
Mass flowmeters (or direct mass flowmeters) have sensing means which respond uniquely to mass flow rate. Other flowmeters employ, for example, sensing means which respond to differential pressure or fluid velocity. If one needs to measure mass flow rate with such devices one must perform separate measurement of density and infer some flow distribution pattern in the cross section of the meter and also infer fluid flow pattern, such as turbulence. They also require Newtonian fluid behavior, which is often not met.
Thus for reason of measurement simplicity alone, the direct mass flowmeters are very desirable. Additionally, other flowmeters generally lend themselves much better to volume flow rate measurement (gallons per minute or liter per second) than to mass flow measurement (tons/hour or kilograms/second). In practice the mass flow measurement is much more useful because chemical reactions require blending of proportional mass (not volume) of ingredients and product specifications mostly refer to mass percentage of ingredients not volume percentage. Thus this represents another major advantage of direct mass flow measurement over other techniques.
Coriolis flowmeters are direct mass flowmeters. They employ the principle of the Coriolis force and use the influence of a pattern of such forces upon a flow tube carrying the fluid within the meter. Devices disclosed to date employ one or two flow tubes which may split the fluid stream and carry a fraction each or may carry the fluid stream serially through both tubes. The flow tubes are typically vibrated by magnetic force coupling between a drive coil and permanent magnet, one or both of which are attached to a flow tube. To permit attachment to outside pipes the end of the flow tubes do not participate in the vibration.
For each part of a flow tube which is momentarily not parallel with the axis of rotation for the element, a Coriolis force is produced. The force acts through the body of the fluid, which will produce pressure on the flow tube wall. The magnitude of the Coriolis force is proportional to the mass flow rate, the angular velocity of rotation and the sine of the angle between flow direction within the element and the direction of the rotation vector.
Under the aggregate of Coriolis forces upon the different parts of the flow tube the flow tube will have motion in addition to the motion caused by the drive (vibrating) motion. "Motion" in this application is used to describe position, velocity, acceleration of a point or aggregate of points on the flow tube or any time-derivative or time-integral of these variables. Over the time of a single flow measurement the flow tube's motion is periodic and any one of these physical variables for almost any point on the flow tube together with known vibrating frequency and amplitude permits determination of the flow rate. The dependency of flow rate determination on drive frequency and amplitude has been of fundamental importance in the design of prior Coriolis mass flowmeters.
When mass flow rate changes the flow tube motion changes which is the principle of the flow measurement. However, if the drive amplitude changes, the Coriolis portion of the motion changes also. If one did not know the new amplitude (for example by absence or inaccuracy of amplitude measurement), the flowmeter may not distinguish a flow rate change from the amplitude change. Similarly, if the fluid temperature changes, the flow tube wall temperature would also change. The elasticity coefficient (Young's modulus) for the flow tube material changes with temperature impacting the Coriolis force induced motion. A change of 10.degree. C. would potentially bring the flowmeter outside specified calibration accuracy (for example 0.2% of reading) if the flow tube was made of stainless steel.
Fluid pressure change modifies the cross section dimension of the flow tube and, thereby, its bending properties. Large pressure changes which may occur in practice can jeopardize calibration accuracy unless the flowmeter design eliminates this hazard.
Major considerations for Coriolis flowmeters are calibration sensitivity and immunity to density change. Process fluids seen by the flowmeter may undergo extensive density change. The reason may be change of fluid temperature and composition. The density change will modify the natural frequency of vibration for the flow tube. Since the flow tubes are usually driven in the immediate vicinity of a natural frequency, the drive frequency will change with density. The flowmeter design determines the extent or complete absence of calibration error due to density shift.
Another problem for Coriolis flowmeters (as well as other types) is entrainment of gases in the fluid. The gas may be in the form of visible or microscopic size bubbles. Gas entrainment causes both density change and change in the coupling between the fluid and the wall of the measurement tube essential for the Coriolis flowmeter. Generally Coriolis flowmeters today exhibit significant to intolerable errors in calibration when the gas entrainment reaches a magnitude of 1% to 3% by volume of gas to volume of fluid.
There is little distinction in principle between Coriolis flowmeters of one or two tube design especially when in the two tube design the tubes are symmetrical and the measurement reference for drive and motion sensing of one flow tube is the other tube. A single flow tube device must use a reference which is not a tube with process fluid. It can be a tube without process fluid, a blade spring or the reference can be the housing itself. A major consideration is mounting requirements to eliminate influence from floor vibrations or pressure pulsation in the process fluid. Another major consideration is that the calibration of the flowmeter does not degenerate excessively when the fluid density changes.
In the past all single flow tube Coriolis mass flowmeters which have had flow tubes with an inside diameter larger than 1/4 inch and have employed a single flow tube design have required extremely complicated mounting. Even after being bolted to a ton of concrete such meters have been reported to be unduly influenced by floor vibration in an ordinary industrial plant. Double flow tube design of similar capacity functions properly in this environment. There is thus an advantage in double tube design.
However, the double tube design has significant drawbacks. It is more costly and may require a flow-splitting section and a flow-combining section made out of cast bodies and having the flow tubes attached by welding. This involves extra cost compared with single flow tube design. It also introduces a physical hazard due to welding attachments which are far more prone to stress corrosion than the flow tube material itself.
Pressure drop is a major factor in many Coriolis flowmeter applications. These meters have become widely used for highly viscous fluids and thick slurries, for example, asphalt, latex paint and peanut butter. In order to keep the pressure drop compatible with the pumping capacity in the line, it is necessary in such applications to work with low mass flow rates. It is also necessary that the flowmeter does not introduce a pressure drop in excess of available pump capacity. In other words, it is of interest to employ a flowmeter with a large diameter and short flow tube but still seeing a low mass flow rate. This may introduce a sensitivity requirement beyond the capacity of all current Coriolis flowmeter designs.
Acoustic waves generated by pumps and other process equipment can cause considerable deterioration of Coriolis flowmeter measurements especially if these waves are periodic and have frequencies in the domain of the drive frequency or the natural frequency of the mode shape closest to the bending caused by the Coriolis forces. Frequent transient random acoustic disturbances may cause similar problems. The flowmeter may lose the ability to distinguish between motion caused by such disturbance from a flow rate change.
The many restrictions stated above are major factors in Coriolis flowmeter design. The present invention addresses all of these restrictions.
Descriptions of Coriolis flowmeters often state that the vibration maintained is at a "natural frequency" of a mode of free (unforced) vibration. In contrast, forced vibrations exhibit the phenomenon of resonance implying maximum response to the driving force. The resonance frequency for a structure of very low damping (typical for most Coriolis flowmeters) is almost the same but not exactly the same as its natural frequency at the proper mode. The magnitude of existing damping and the method of forcing determine the difference. Since all Coriolis flowmeters are exposed to some force to maintain the vibrations, there must necessarily be a difference between the exact natural frequency and the one which is actually achieved. In order to simplify the description this small distinction will be ignored and where disclosures have been made stating that the structures are vibrated at a "natural frequency" this will be considered equivalent with operation at resonance with some selected mode.
B. Prior Patents
The advantages of the Coriolis flowmeter principle has stimulated the development of many patents. Among them are the following:
U.S. Pat. No. 3,329,019 (Sipin) discloses two straight single tube Coriolis mass flowmeter embodiments. The preferred embodiment (FIGS. 2, 3, 4 of the patent) employs a driving force in the center of a uniform flow tube using a fixed frequency, mechanical drive. The bending produced by the drive motion is of single polarity along the whole flow tube at every instant of operation (ignoring the effect of Coriolis force bending). The flow tube is semi-pinned at the ends with blade-springs. Strain gauges on these springs sense the tube motion and the difference between the strain gauge signals is computed by a bridge circuit. This difference represents flow and is displayed on a meter.
One major drawback of the Sipin meter is that the mechanical drive forcing vibration of a low-damped mechanical system will introduce complex signal waveforms with resulting poor measurement sensitivity. The sensitivity of measurement using that drive bending pattern is low due to relatively low angular velocities produced along the beam with small Coriolis force magnitudes. If sensitivity is enhanced by driving near the first mode natural frequency, the mechanical system would become excessively irregular causing huge disturbance levels on the signals. A further drawback is that the meter is highly sensitive to amplitude and frequency of the drive motion making overall performance prohibitively low. Another drawback is the seals which are employed for attaching the flow tubes to the meter body in order to give the springs freedom of action. Under process conditions such seals would be prone to leak.
The major differences between this Sipin device and the present invention are:
1. The present invention drives the beam in resonance with a higher anti-symmetric frequency mode. This requires much less driving force and power consumption and also furnishes smooth operation.
2. The present invention drives the beam with opposing polarity on each half of the beam at every instant in time. The preferred embodiment of the invention uses a nonuniform beam cross section profile and mass distribution. Combined with feature (1) this results in an order of magnitude higher sensitivity. This design is insensitive to drive amplitude and frequency changes.
The second embodiment in the Sipin patent (FIGS. 7, 8, 9 of the patent) employs a single, straight, uniform flow tube which is attached with bellows to inlet and outlet pipe sections. It is vibrated in the center with an electromagnetic drive at constant frequency. The drive motion represents a single polarity waveform along the tube at any instant in time. Two coil/magnet sensors mounted on the flow tube near the bellows produce velocity signals. These signals are fed to two amplifiers, one of which forms the sum, the other the difference between the velocity signals. The difference is employed as a measure of the makss flow rate. The sum is a measure of drive amplitude and is used in a feedback control for drive amplitude control.
The second Sipin embodiment has reduced calibration sensitivity to drive amplitude due to feedback control. However, the flowmeter calibration accuracy will depend on the stability of the control loop to hold the amplitude within the performance specifications typically required for Coriolis flowmeters. Furthermore, the meter is sensitive to the constancy of the drive frequency regulation and unavoidable inaccuracy will directly impact flowmeter calibration accuracy. The meter depends on a soft bellows for obtaining sensitivity. Bellows tend to introduce very large calibration sensitivity to fluid static pressure change and this would create a major practical problem. With harder bellows the sensitivity of this design pattern is much less than the present invention.
The differences between the second Sipin embodiment and the present invention are:
1. The present invention uses opposing polarity of drive position and velocity for each half of the beam. This invention measures the response in areas of the beam where the wave shape of the Coriolis force induced motion is similar to the bending mode for the lowest natural frequency giving far higher sensitivity. This invention is fundamentally independent of both drive amplitude and frequency regardless of the accuracy or absence of control of either variable. The preferred embodiment of this invention uses a nonuniform cross section profile and mass distribution. This invention does not employ a bellows.
U.S. Pat. Nos. 3,355,944 and 3,485,098 (Sipin) show single tube meters with some curvature as well as full shaped U-tubes in different embodiments. All employ a central drive, implying single polarity deviation of the flow tube from equilibrium at all times. Again this is exactly opposite to the philosophy of the present invention. An embodiment without bellows is shown. These embodiments depend on flow tube relaxed position curvature.
U.S. Pat. No. 4,109,524 (Smith) discloses a three section flow tube with a center section connected with bellows to the outer sections. Each section is straight and uniform and all three have coinciding central axes. A fixed frequency, mechanical drive in the center moves the sections so that at any instant of time the deflection from the center position is of the same polarity for all points on all three sections. This is opposite to the present invention. The Smith design employs force balance repositioning of the center beam. The torque is measured each time the central beam passes through its central position. The magnitude of that torque is employed as a measure of the mass flow rate. U.S. Pat. No. 4,109,524 has the same differences from the present invention as the meters of the Sipin patents. Additionally, the preferred embodiment of the present invention does not employ torque measurement and it does not perform "snapshot observation" at a central position.
U.S. Pat. No. 4,127,028 (Cox & Gonzales) discloses a double flow tube meter design. The flow tubes are of identical shape and construction. Both are U-shaped but with the legs of the U drawn together. They are mounted in parallel, cantilevered fashion on a fixed mount. The fluid flows through in the same direction through both U tubes. The drive is electromagnetic with a drive coil on one U (at the bottom point of the bight end) and a magnet on the other U in the equiresonance frequency associated with the lowest frequency mode with vibration around the mounting point. The Coriolis forces twist the U tubes so that they are no longer plane. This motion element corresponds to another vibrating mode. The U tubes are shaped so that the natural frequency of the response mode is nearly the same as the drive resonance frequency. The objective is to enhance the response magnitude. A drawback not discussed by the patentees is that the response will become dependent on the natural damping which is introduced by fluid/flow tube interaction and internal crystal motion in the tube walls. Coil/magnet sensors measure the velocities of the sidelegs of the U tubes with respect to each other at chosen symmetrical locations on each leg.
The Cox & Gonzales meter is similar to the present invention in the use of forced vibration at a natural frequency and a structural design where the response deflection pattern is predominantly in a mode which does not have a widely different natural frequency. Major differences between the disclosure and a basic aspect of the present invention are that the invention uses substantially straight flow tubes rather than U tubes and that the invention forces vibration in the second (or higher) mode not the lowest mode with respect to rotation around the mounts. Finally the present invention is independent of changes in damping.
U.S. No. Re 31450 (Smith) covers a commercial product, now withdrawn, produced by Micromotion Inc. The primary embodiment (FIGS. 1-8 of the patent) employs a single U-shaped flow tube with cantilever attachment to a fixed mount at the ends of the flow tube. An electromagnetic drive using peak detector feedback vibrates the flow tube with parallel drive motion of each leg of the U except for the influence of Coriolis forces. The drive coil is mounted on a blade spring and the drive magnet on the flow tube. The blade spring is manually adjusted to match the natural frequency of the flow tube in its first vibrating mode when the flow tube is filled with a fluid of a particular density. If the fluid had a different density, a different adjustment (by weight modification) must be done. The flow tube is vibrated at the resonance frequency of its first natural mode of vibration. Optical sensors measure the twist of the U tubes at symmetrical points on the legs similar to Cox & Gonzales. The sensor signals are an on/off-type using the effect of a shadow of blades mounted on the legs. When the shadow of the first leg arrives at the relaxed position (midplane) it triggers a photodetector to start a counter. When the other leg triggers its photodetector, the count is taken. The time differential for the midplane arrival is used for proportional determination of the mass flow rate. The tiwsting motion of the U coincides with the motion occurring during free natural vibration in mode with higher frequency than the drive frequency.
The main drawback of the method described by U.S. No. Re 31450 is that it depends on the housing as position reference. Any gradual warping of this housing due to stress release or other reason would bring in a sustained calibration shift. The instrument is extremely sensitive to floor and outside pipe vibrations and requires rigid attachment to huge weights such as concrete blocks. The assumption of linear relationship between flow rate and midplane time differential applies only for small time differentials which restricts the range of measurement. The design is also sensitive to fluid density change and natural frequency change due to changes in fluid temperature and pressure. Drift of the feedback-controlled drive amplitude will also cause calibration errors.
U.S. Pat. No. 4,422,338 (Smith) covers a meter sold by Micromotion Inc. under the name C-Model. This patent discloses a design which is, in principle, identical to the preferred embodiment of U.S. No. Re 31450. The difference is that the optical sensors have been replaced by magnet/coil velocity sensors and that the blade spring for drive counterbalance has been replaced by an empty flow tube. The signals from the velocity sensors are integrated and amplified into square waveform. This provides waveform and phase identity with the signals produced by the optical "shadow sensors" employed in the No. Re 31450 design. The signals are used in the same manner to determine the difference in arrival time at the midplane. The drive system in this disclosure is the same as disclosed in No. Re 31450. In order to eliminate drift which could (and does) occur in the integrators, these circuits are limited to the frequency domain of the drive frequency and no integration is performed at a low or zero frequency. Except for the freedom from gradual shift in housing position this design has all the other drawbacks pointed out for the design of No. Re 31450.
Major differences between the two Smith patents just described and the present invention are as follows: First, an aspect of this invention uses substantially straight flow tubes, not U tubes. This invention drives the flow tube at a resonance frequency corresponding to a high anti-symmetric mode not to its first mode. This invention does not employ any time measurement in its preferred embodiment. In an alternative embodiment, this invention uses time measurement for phase detection without reference to a midplane location. In this embodiment this invention does not assume linear relationships. This invention in its preferred embodiment does not use a blade spring or empty flow tube as counterbalance in the drive force application. This invention uses a detection method which eliminates all sensitivity to fluid density, inaccuracy in drive amplitude regulation, and drive frequency change.
U.S. Pat. No. 4,491,025 (Smith & Cage) describes meters that are sold by Micromotion Inc. under the designation "D-Model". The meters use two U tubes similar to Cox & Gonzales but the legs are straight in the vicinity of the mount and over most of the sides of the tubes. It uses an electromagnetic drive acting between the tubes creating relative motion. Magnet/coil velocity sensors determine the relative motion between the sensors as in the Cox & Gonzales design. Some elements of the meters of U.S. Pat. No. 4,422,038 are incorporated in the preferred embodiment of U.S. Pat. No. 4,491,025, namely the integrators and amplifiers which convert the resulting position signals to square waves. These square waves control a counter for measurement of time differential at arrival at "the respective midplanes" of the tubes. The devices sold by Micromotion which are stated to be covered by this patent do not have any feature for "midplane" or "relaxed position" determination but simply trigger the counter for timing when the integrated velocity signals pass at some preset and constant deviation from zero. The patent also discloses "plenums" or two small chambers which are provided at the inlet and outlet of the flow tubes. The fluid is split into two streams and recombined with a plenum handling each of those functions. No physical mechanism explaining how the plena improve the operation is furnished. They do not allow wave bypass or attenuation by a tuned hydraulic circuit as disclosed in the present application.
The meter of U.S. Pat. No. 4,491,025 has the same limitations as that of U.S. Pat. No. 4,422,338 except that the anchoring of the flowmeter to a huge mass of material is not necessary.
U.S. Pat. No. 4,559,833 (Sipin) describes a commercial Coriolis flowmeter sold by Smith Meter Company. It employs single or double, parallel flow tubes in different embodiments. The flow tubes are S-shaped. The drive force is applied at the center of the S. Sensing devices are mounted near the top and bottom of the S. In one embodiment the sensors are optical on/off switches and difference in arrival time at a fixed position is used as a measure of flow rate. Another embodiment employs analog deflection sensors and the difference of the two transducer signals is used to determine the flow rate. A counterbalancing spring for the drive is also presented as an additional embodiment of the drive system.
In contrast to U.S. Pat. No. 4,559,833, the present invention does not use an S-shaped conduit as a flow tube. In the preferred embodiment, this invention uses the difference in separately located motion sensor signals. However, the present invention creates a unique previously undetected advantage by a particular combination of such differential signals with other mathematical operation. This gives major advantages over the design disclosed in U.S. Pat. No. 4,559,833 in terms of independency to density, drive frequency and amplitude shifts.
U.S. patent application Ser. No. 775,739 (in the name of the present applicant) describes commercial products sold by Exac Corporation. That application describes single and multi flow tube design where each flow tube has a helical design (cross-over loop). The drive vibration is at the lowest resonance frequency of the structure. The Coriolis forces twist the loop and produce a response predominantly in the third mode of natural vibration. In one embodiment sensors on each side of the loop are used for differential phase measurement using nonlinear relationship including tangent function. This embodiment requires determination of drive frequency to be used in the measurement algorithm. It employs a temperature sensor attached to the flow tube to furnish compensation for fluid temperature change. Another disclosed sensing embodiment uses a position or velocity measurement between two tube sections at the crossover point. Electromagnetic dampeners are presented for restricting loop vibrations. A disclosure is made of a velocity feedback control loop for continuous feedback regulation of loop velocity in the direction of the drive.
U.S. Pat. No. 4,660,421, issued to Dahlin et al., also describes commercial products sold by Exac Corporation. The application expands on a special version of the helical loop design described in Ser. No. 775,739. The latter discloses a general helical loop which might have a circle as projection on a certain plane or have a projection of any other shape. The former designates a projected shape with elongation in the direction of the opposed situated inlet and outlet flanges. It also shows the usage of a horseshoe magnet and coil as an embodiment of local velocity sensing.
The present invention is different from the meters described in the referenced U.S. patent applications due to the absence of crossover loops. The present invention operates with a drive frequency which is the resonance frequency for a higher mode than the mode in which the response to the Coriolis forces occurs, which is opposite to the teachings of Ser. No. 775,739 and U.S. Pat. No. 4,660,421. The instant invention accomplishes in its preferred embodiment frequency, independency without the complexity of explicit measurement of that frequency. This invention is also fundamentally independent of frequency instead of relying on a particular approximate formula for adjustment to frequency change. The present invention has an additional advantage over the devices of Ser. No. 775,739 and U.S. Pat. No. 4,660,421, namely that it has fundamental independency to drive amplitude shift in contrast to an independency which is valid only as long as constancy in pulse wave form is maintained.
C. Related Product Literature
The following commercial product literature discloses technology related to the present invention.
Hewlett-Packard Application Note No. 200-3 (1974) entitled "Precisiontime Interval Measurement Using an Electronic Counter". This document describes a standard product for determining the phase angle between two periodic electronic signals having the same frequency. It uses the time difference between zero crossing of the two signals determined by a counter started and stopped by the leading and trailing signal, respectively. It also determines by separate counting the frequency itself and obtains the phase angle by division.
Danfoss Type Mass 1000/1100 Mass Flowmeter (brochure printed 1985). This document, which relates to U.S. Pat. No. 4,680,974 issued to Simonsen et al., describes a Coriolis flowmeter using two slightly curved uniform flow tubes. The process flow stream is divided with each flow tube carrying a fraction. The flow tubes are rigidly attached at both ends adjacent to each other. An electromagnetic drive with a drive coil is mounted at the center of one flow tube and a magnet is mounted opposite to it and in the center of the other flow tube. The drive operates at the resonance frequency of the first mode of vibration. Magnet/coil type velocity sensors mounted at about 1/4 and 3/4 distance from the end determine the combined motion due to drive and Coriolis forces. The sense coils are mounted on one flow tube and the sense magnets on the other and thus the device detects relative motion similar to Cox and Gonzales. The time differential between arrival at zero velocity seen at the two sensor locations is employed as a measure of flow rate. The time differential is determined by an electronic counter. The deflection due to Coriolis forces is in a modeshape of higher frequency than the drive frequency.
The major differences between the Danfoss design and the instant invention are as follows: The present invention in one of its preferred embodiments uses one flow tube, not two. This invention drives the flow tube at the resonance frequency corresponding to a higher anti-symmetric mode rather than the first mode. The instant invention measures the response in a far more sensitive deflection pattern. Combined with this advantage the present invention applies a unique nonuniform flow tube cross section and mass distribution for further enhancement of sensitivity. The combined result is one to two orders of magnitude higher sensitivity with this invention than the Danfoss design deriving from flow tube design and operation only. Furthermore, this invention employs a unique signal handling method leading to immunity to fluid density, drive frequency and amplitude shift.
Solartron Transducers, Houston, Tex. Brochure: Liquid Density Transducers Type 7830 and 7840. This brochure describes densitometers which use one vibrating flow tube for its primary sensing function. The natural frequency of vibration or the associated nearby resonance frequency for forced vibration is a function of the fluid density. Larger mass within the fixed volume confinement of the flow tube makes the tube vibrate slower. This densitometer is stated to operate "at one of its natural frequencies" and is stated to "overcome the normal difficulties associated with vibrating a single straight tube". This must be interpreted to imply a drive frequency at a higher mode than the first mode since many other densitometer disclosures have operation in the first mode. The present invention also uses a drive in a higher frequency mode and uses a straight single tube. However, a totally different set of problems originates in the design of a Coriolis flowmeter than in a vibrating densitometer. The reasons and some of the major motivation in the flowmeter design for using the higher mode drive frequency is the need for sensitivity in detecting influence of Coriolis forces, which are not essential and probably a nuisance for densitometer design. In order to make a higher mode Coriolis flowmeter successful, different understanding and design considerations come into the forefront which have no relevance for the densitometer design. The enhancement of flowmeter response by nonuniform cross section profile and mass distribution presented in this invention would not be meaningful for the densitometer design.